The invention relates to a communication system and, more particularly, to a communication system used for a local network.
Recently, local networks which have interconnected computer units distributed in relatively small areas are extensively used. An Ethernet system proposed by Xerox Company is a typical example. The system adopts a common bus structure using a linearly extending coaxial cable as the transmission medium. Various local networks have been developed other than the Ethernet system. At present, networks of different systems cannot be interconnected. The International Standardize Organization (ISO) has been working on a standization plan for the interconnection of networks since 1977. This plan is called Open Systems interconnection (OSI). The OSI consists of seven protocol stages called physical level stage, data link stage, network stage, transport stage, session stage, presentation stage and application stage from the lowest stage in the mentioned order. The physical level stage prescribes electrical and mechanical conditions for setting, maintaining and cancelling physical inter-system lines. An example of this protocol stage is X.21, V.24, V.35 prescribed by CCITT. The data link stage provides an inter-system data link, through which the data transfer and error correction are controlled. An example of this stage is HDLC or X.25 (level 2) protocol. In the Ethernet system, only these two protocol stages are prescribed, and the other protocol stages are not. The Ethernet system does not prescribe the flow control and acknowledgment of transfer as a text frame transferred from the upper protocols. It leads to the reduction of effective transfer rate and response time.
The effective transfer rate and response time are also reduced for the following reason. In the local network only a single frame can be transmitted at a time on a bus line which consists of a coaxial cable or the like. The network access from each node i.e., junction of bus and each computer unit is controlled by a carrier sense multiple access/collision detection (CSMA/CD) system. According to this system, each node can start transmission if there is no frame transmitted from the other nodes to the bus, i.e., if the bus is silent. If there is a frame on the bus, the transmission is delayed until the frame vanishes. However, a plurality of nodes can simultaneously start transmission when these nodes detect the silence state of the bus. In such a case, collision of a plurality of frames can occur on the bus. In order to avoid such a collision, in the CSMA/CD system the transmitter node checks the state of the bus after the frame transmission has been completed. If a collision is detected, the subsequent transmission is postponed for a suitable period of time. The waiting period is set for each node by making use of random numbers. Thus, a collision rarely occurs again when the transmission is started after the lapse of the waiting time. With this arrangement of the Ethernet system (hereinafter referred to as original Ethernet system), however, the response time, and hence the effective transfer rate, is further reduced.
An Acknowledging Ethernet system proposed by Keio University in Japan is an improved version of the original Ethernet system. In this system, the acknowledgement of transfer which has been effected in an upper protocol in the original Ethernet system, is done in the data link stage. More particularly, the preparation and transmission of an acknowledgement frame is undertaken not by the host side but by the node side controller, thereby reducing the burden on the host side and reducing the operation speed. Further, higher priorities are given to the transmission of the acknowledgement frame than to the transmission of the general text frame. This is realized by setting the period, which is provided from the instant of detection of the silence state till the start of transmission of the acknowledgement frame, to be shorter than the period till the start of transmission of the text frame. However, in the Acknowledging Ethernet system the bus utility efficiency is decreased because a fixed period always has to be provided from the instant of detection of the silence state till the start of transmission of a text frame.